The more than 50-year-old mystery of how fluoride battles tooth decay may be one step closer to being solved, researchers say.

The study suggests that fluoride works by reducing the ability of bacteria to stick to teeth, making the germs easier to wash away with saliva, brushing and other activities.

When fluoride bonds with tooth enamel, bacteria probably cannot hold onto it as strongly, said study researcher Karin Jacobs, a physicist at Saarland University in Germany.

Fluoride is now often added to drinking water, toothpastes and mouthwashes. Although fluoride compounds reduce the risk of cavities, despite more than a half-century of research, it remains controversial exactly how they do so.

Fluoride’s cavity-fighting benefit is often explained by its ability to fuse with teeth to create an acid-resistant layer. However, there is also evidence that fluoride might control mouth bacteria, which cause tooth decay.

Analyzing real teeth can be tricky, because teeth can differ substantially from one another, and even a single tooth can vary throughout its composition. This variability helps explain the decades of difficulty in figuring out exactly how fluoride keeps bacteria off teeth.

Scientists have tried experimenting on artificial versions of the main ingredient of tooth enamel, a compound known as hydroxyapatite. However, this material is often more porous than real enamel, so it is not a good representation of a real tooth’s composition.

To create better tooth mimics, Jacobs and her colleagues polished artificial teeth with microscopic grains of diamond to make the surface as smooth as possible. Some tooth imitations were then exposed to fluoride, while others were not. Both groups were prodded with probes covered in tooth-decay germs, and examined under a microscope to measure how well the bacteria clung.